Let's say you're playing seven-card stud. You have a pair
of jacks, and on fifth street your opponent bets. You know
he has a big hand. So your response is easy: You fold. Suppose
you know your opponent is bluffing with nothing. Again your
response is easy: You raise. Suppose you think he has you
beat with two small pair, but you're getting sufficient
pot odds for a call. So you call. Straightforward bets,
straightforward responses.

But what if your opponent is not so straightforward? What
if he's the kind of player who might be betting with a legitimate
hand but might also be semi-bluffing? He's not always semi-bluffing,
of course. That would also make it too easy to respond,
because if you know an opponent is semi-bluffing when he
bets, you can simply raise with anything, and he will probably
fold. The problem arises when you think an opponent may
be semi-bluffing but can't be sure he does not have a legitimate
hand. What's more, if he doesn't have a legitimate hand
now, he may get it later - or he may look like he's gotten
it later.

It turns out there aren't many defenses against the semi-bluff,
which is why it is such a powerful play. Frequently the
best play against a possible semi-bluff is to fold, especially
when the pot is small. All right, your opponent has beaten
you. He may even have made you throw away the best hand.
But if you call his bet, he has three other ways of beating
you. He may in fact have had the best hand when he bet.
He may have been semi-bluffing, but he now outdraws you.
Or he may have been semi-bluffing, but he proceeds to catch
scare cards that force you to fold. Therefore, though you
may have thrown away what was the best hand at the moment,
still your opponent had too many ways of beating you to
justify your calling his bet.

Even when you think you are favored to have the best hand,
it may be correct to fold. Let's say you think it's a little
better than even money that your opponent is semi-bluffing.
For convenience, we'll say you think there's a 52 percent
chance he's semi-bluffing and a 48 percent chance he has
a good hand. If he is semi-bluffing, you figure you're a
6-to-5 favorite to beat him. However, if he isn't semi-bluffing
and has the hand he's representing, you're virtually locked
out. Thus, 52 percent of the time you're a favorite to win.
Should you call his bet? Many professionals as well as amateurs
make the mistake of calling in such situations, but unless
the pot is large, the correct play is to fold.

Let's work it out mathematically. You lose almost automatically
48 percent of the time. Of the remaining 52 percent, you'll
win an average of six out of 11 hands (since you estimate
yourself to be a 6-to-5 favorite). In other words, you'll
lose almost half the time when you're a slight favorite
and virtually all of the time when you're a big underdog.
You stand to win the hand only 29 percent of the time in
all. To call the bet then, you would need to be getting
at least 7-to-3 effective odds from the pot, which is not
very likely in an early betting round. Hence, the correct
play would normally be to fold.